对实数x,y,求S=x2+2xy+3y2+2x+6y+4的最小值

把等式看成是x的方程配方S=x²+2（y+1）x+3y²+6y+4S=x²+2（y+1）x+(y+1)²-(y+1)²+3y²+6y+4S=（x+y+1）²-y²-2y-1+3y²+6y+4S=(x+y+1)²+2y²+4y+3S=(x+y+1)²...